Rails Security
Jonathan Weiss, 30.10.2009
Peritor GmbH

Who am I ?
I work at Peritor in Berlin
I tweet at @jweiss
I code at http://github.com/jweiss
I blog at http://blog.innerewut.de
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Peritor
Working on
http://scalarium.com
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Agenda
Follow the application stack
and look for
Setup and deployment
• Information leaks
Application code
• Possible vulnerabilities
• Security best practices
Framework code
Rails Application Stack
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Rails Application Setup
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Rails Setup
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Rails Setup - FastCGI
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Rails Setup - Mongrel
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Rails Setup – mod_rails
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Rails Setup – Unicorn
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Information leaks
and
vulnerabilities
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Information leaks
Is the target application a Rails application?
• Default setup for static files:
/javascripts/application.js
/stylesheets/application.css
/images/foo.png
• URL schema
/project/show/12
/messages/create
/folder/delete/43
/users/83
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Information leaks
Is the target application a Rails application?
• Rails provides default templates for 404 and 500 status pages
• Different Rails versions use different default pages
• 422.html only present in applications generated with Rails >= 2.0
• Dispatcher files not present in recent Rails versions
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Sample Status Pages
http://www.twitter.com/500.html http://www.43people.com/500.html
http://www.engineyard.com/500.html Rails >= 1.2 status 500 page
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Server Header
GET http://www.haystack.com
Date: Wed, 28 Oct 2009 11:23:24 GMT
Server: nginx/0.6.32
Cache-Control: max-age=0, no-cache, no-store
…
GET https://signup.37signals.com/highrise/solo/signup/new
Date: Wed, 28 Oct 2009 11:54:24 GMT
Server: Apache
X-Powered-By: Phusion Passenger (mod_rails/mod_rack) 2.2.5
Status: 200 OK
…
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Server Header
GET http://www.twitter.com
Date: Wed, 28 Oct 2009 11:23:24 GMT
Server: hi
Status: 200 OK
…
GET http://www.golfermail.org
Date: Wed, 28 Oct 2009 11:13:41 GMT
Server: Mongrel 1.1.5
Status: 200 OK
…
Disable Server header
# httpd.conf
Header unset Server
Header unset X-Powered-By
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Information leaks
Subversion metadata
• Typically Rails applications are deployed with Capistrano / Webistrano
• The default deployment will push .svn directories to the servers
GET http://www.strongspace.com/.svn/entries
…
dir
25376
http://svn.joyent.com/joyent/deprecated_repositories/www.strongspace/trunk/public
http://svn.joyent.com/joyent
Prevent .svn download
2006-04-14T03:06:39.902218Z <DirectoryMatch "^/.*/.svn/">
34 ErrorDocument 403 /404.html
justin@joyent.com Order allow,deny
Deny from all
…
Satisfy All
</DirectoryMatch>
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Cookie Session Storage
Since Rails 2.0 the session data is stored in the cookie by default
Base64(CGI::escape(SESSION-DATA))--HMAC(secret_key, SESSION-DATA)
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Cookie Session Storage
Security implications
• The user can view the session data in plain text
• The HMAC can be brute-forced and arbitrary session data could be created
• Replay attacks are easier as you cannot flush the client-side session
Countermeasures
• Don’t store important data in the session!
• Use a strong password,
Rails already forces at least 30 characters
• Invalidate sessions after certain time on the server side
… or just switch to another session storage
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Cookie Session Storage
Rails default session secret
Set HTTPS only cookies
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Cross-Site Scripting - XSS
“The injection of HTML or client-side Scripts (e.g. JavaScript) by malicious users into
web pages viewed by other users.”
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Cross-Site Scripting - XSS
Cases of accepted user input
• No formatting allowed
search query, user name, post title, …
• Formatting allowed
post body, wiki page, …
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XSS - No Formatting Allowed (Rails 2.x)
Use the Rails `h()` helper to HTML escape user input
But using `h()` everywhere is easy to forget.
Better, use safeERB, XSS Shield, or rails_xss plugins:
http://agilewebdevelopment.com/plugins/safe_erb
http://code.google.com/p/xss-shield/
http://github.com/NZKoz/rails_xss
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XSS - No Formatting Allowed (Rails 3)
Rails 3 auto escapes strings in RHTML template
Explicitly mark strings as HTML safe
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XSS - No Formatting Allowed (Rails 3)
rails_xss Plugin
• Build-in in Rails 3
• Introduces “Safe Buffer” concept
• Updates all the helpers to mark them as html_safe!
• Requires Erubis
Install and get familiar with if on Rails 2.x
http://github.com/NZKoz/rails_xss
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XSS - Formatting Allowed
Two approaches
Use custom tags that will translate to HTML (vBulletin tags, RedCloth, Textile, …)
Use HTML and remove unwanted tags and attributes
• Blacklist - Rails 1.2
• Whitelist - Rails 2.0
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XSS - Custom Tags
Relying on the external syntax is not really secure
Filter HTML anyhow
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XSS - HTML Filtering
Use the Rails `sanitize()` helper
Only effective with Rails > 2.0 (Whitelisting):
• Filters HTML nodes and attributes
• Removes protocols like “javascript:”
• Handles unicode/ascii/hex hacks
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XSS - HTML Filtering
sanitize(html, options = {})
http://api.rubyonrails.com/classes/ActionView/Helpers/SanitizeHelper.html
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XSS - HTML Filtering
Utilize Tidy if you want to be more cautious
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Session Fixation
Provide the user with a session that he shares with the attacker:
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Session Fixation
Rails uses only cookie-based sessions
Still, you should reset the session after a login
The popular authentication plugins like restful_authentication are not doing this!
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Cross-Site Request Forgery - CSRF
You visit a malicious site which has an image like this
Only accepting POST does not really help
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CSRF Protection in Rails
By default Rails > 2.0 will check all POST requests for a session token
All forms generated by Rails will supply this token
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CSRF Protection in Rails
Very useful and on-by-default, but make sure that
• GET requests are safe and idempotent
• Session cookies are not persistent (expires-at)
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SQL Injection
The user’s input is not correctly escaped before using it in SQL statements
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SQL Injection Protection in Rails
Always use the escaped form
If you have to manually use a user-submitted value, use `quote()`
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SQL Injection Protection in Rails
Take care with Rails < 2.1
Limit and offset are only escaped in Rails >= 2.1
( MySQL special case )
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JavaScript Hijacking
http://my.evil.site/
JSON response
The JSON response will be evaled by the Browser’s JavaScript engine.
With a redefined `Array()` function this data can be sent back to http://my.evil.site
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JavaScript Hijacking Prevention
• Don’t put important data in JSON responses
• Use unguessable URLs
• Use a Browser that does not support the redefinition of Array & co,
currently only FireFox 3
• Don’t return a straight JSON response, prefix it with garbage:
The Rails JavaScript helpers don’t support prefixed JSON responses
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Mass Assignment
User model
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Mass Assignment
Handling in Controller
A malicious user could just submit any value he wants
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Mass Assignment
Use `attr_protected` and `attr_accessible`
Vs.
Start with `attr_protected` and migrate to `attr_accessible` because of the different
default policies for new attributes.
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Rails Plugins
Re-using code through plugins is very popular in Rails
Plugins can have their problems too
• Just because somebody wrote and published a plugin it doesn’t mean the plugin is
proven to be mature, stable or secure
• Popular plugins can also have security problems, e.g. restful_authentication
• Don’t use svn:externals to track external plugins,
if the plugin’s home page is unavailable you cannot deploy your site
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Rails Plugins
How to handle plugins
• Always do a code review of new plugins and look for obvious problems
• Track plugin announcements
• Track external sources with Piston, a wrapper around svn:externals
http://piston.rubyforge.org/
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Conclusion
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Conclusion
Rails has many security features enabled by default
• SQL quoting
• HTML sanitization
• CSRF protection
The setup can be tricky to get right
Rails is by no means a “web app security silver bullet” but adding security
is easy and not a pain like in many other frameworks
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Questions?
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Peritor GmbH
Blücherstaße 22
10961 Berlin
Telefon: +49 (0)30 69 20 09 84 0
Telefax: +49 (0)30 69 20 09 84 9
Internet: www.peritor.com
E-Mail: kontakt@peritor.com
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